The invention relates to removal of toxic substances from water, and is particularly directed to removal of toxic metals or other toxic elements from industrial waste water as well as from agricultural tile drain water. Typically, such substances occur naturally in low concentrations but become concentrated through evaporation or bioaccumulation. Such substances include selenium.
The subject matter of this invention is related to that of U.S. Pat. No. 4,940,549, issued to the same assignee Jul. 20, 1990 and incorporated herein by reference. The patent discloses a process for removing selenium and other toxic substances/metals from agricultural tile drain water. Selenium is technically not a metal, but is sometimes referred to as an "amphoteric metal".
The process of the present invention is particularly concerned with removal of selenium from the effluent waste waters produced from petroleum refineries, although it is useful with other industrial effluent waters as well. Industrial processes include coal washing and many metallurgical processes, particularly precious metals process solutions after cyanide oxidation. The latter includes mine drainage and mineral processing waste, and particularly heap leach gold mine washings. In the case of some industrial processes there will also be, or there will alternatively be, molybdenum and/or other toxic metals present. The process of the invention is effective to remove most such toxic metals, as well as selenium.
In this specification and the accompanying claims, the term "selenium" is intended to refer to all soluble compounds of this element, as well as the elemental form itself.
A particularly acute problem of concentrated selenium discharge occurs in the waste waters from petroleum refineries. Many refineries have this problem, to a lesser or greater extent depending on the origin of the crude oil. As the selenium is isomorphous with sulfur, it accompanies sulfur in the processing of the oil. If selenium is present it generally accompanies sulfur in the oil. Crude oil taken from the San Joaquin Valley in California, for example, has a relatively high selenium content. In the waste water discharge from refineries processing this crude oil, selenium content has been high, above accepted levels of toxicity in San Francisco Bay where several refineries are located.
The selenium content of the effluent refinery liquor, at least in the case of Unocal's San Francisco refinery, originated from a combination of liquid streams of the refinery's segregated phenolic water system. Those streams included effluent from hydrotreating/reforming equipment, crude distillation and coking, hydrocracking, sulfur recovery plants and other miscellaneous small streams, placed in a common storage area. The various streams were mixed together in a common effluent, which had an average range of selenium from about 2.1 to about 6.1 parts per million or milligrams per liter. As discussed further below, the sample treated in testing in accordance with the invention had a selenium content of 4.87 parts per million.
Usually some treatment is performed on refinery effluent before it is discharged to a river or bay, sometimes granular carbon filtration. Most of the selenium remains in the effluent, including a large amount in suspension as well as that in solution.
Waste water effluent from the Unocal Corp. refinery in Rodeo, Calif. (discharged into San Francisco Bay) is typified by the data given in Table I.
TABLE I __________________________________________________________________________ UNOCAL CORP. REFINERY WASTE WATER ANALYSIS (San Francisco Refinery, Rodeo, California) (PHENOLIC WATER ANALYTICAL DATA Parameter Flow .+-. 3500 bpd (= 102 gpm) pH 7.0-8.8 Concentration Concentration (ppm) Parameter (ppm) Parameter Filterable Total __________________________________________________________________________ COD 4400-8100 Ca (CaCO.sub.3) 5-15 5.5-16 Mg (CaCO.sub.3) 2.4-8.9 2.6-9.7 Cyanides 3.0-18 Na (CaCO.sub.3) 1400-1900 1400-1900 Phenols 80-158 Ni 0.1-0.6 0.2-1.29 H.sub.2 S 0.0-0.7 K 1.5-19 1.5-19 As -- 0.03-0.1 NH.sub.3 23-121 Cr -- 0.01-0.16 Cu 0.02-0.08 0.02-0.39 N (total) 158-204 Fe 0.93-3.5 1.6-4.1 Se -- 2.1-6.06 Carboxylic Acids 340-400 Sr 0.02-0.07 0.02-0.07 Ti -- 0.03-0.14 Thiosulfate 860-1200 V 0.02-0.1 0.02-0.15 Thiocyanate 31 Zn 0.02 0.02-0.13 Cyanate 2 Al 0.2 0.005-1.2 Ag -- &lt;0,002-0.002 Pb &lt;0.05 &lt;0.05 Mn &lt;0.01-0.02 &lt;0.01-0.03 B 1.1-2.1 0.1 Float Oil 0.5-25.0 P -- &lt;0.1-0.7 Si(SiO.sub.2) 12-18 -- S 110-530 120-540 Cl(CaCO.sub.3) 110-130 -- SO.sub.4 (CaCO.sub.3) 66-120 -- NO.sub.3 (CaCO.sub.3) 3 -- NO.sub.2 &lt;1-&lt;5 -- Ba &lt;0.1 &lt;0.1 Mo &lt;0.1 &lt;0.1 Sb &lt;0.1 &lt;0.1 Cd &lt;0.01 &lt;0.01 Co &lt;0.01 &lt;0.01 Zr &lt;0.01 &lt;0.01 __________________________________________________________________________ *Compiled from 1985-1991 data Prior selenium removal systems and method have been concerned primarily with mine waters and similar drain waters. Many efforts have been made, unsuccessfully, to remove selenium from agricultural drain waters (except as reported in U.S. Pat. No. 4,940,549) and from industrial effluent waters such as refinery waste water. It is known that selenium plus 6 valence can be reduced to selenium plus 4, which occurs as selenite. This can be reduced to elemental selenium at a valence of 0. This can be reduced further to a valence of minus 2 or selenide. A complicating factor in the agricultural drain waters (mentioned in U.S. Pat. No. 4,940,549) is the presence of nitrates originating from nitrogenous fertilizers, but nitrates are essentially absent from refinery waste water.
U.S. Pat. No. 4,405,464, issued to Kerr-McGee Nuclear Corporation, is pertinent to this invention in that it discloses a process for removing selenium from mine waters. The Kerr-McGee process involves passage of the liquid solution up through a column of iron particles. This would involve a very large amount of iron, with a limited amount of water passing through, and the process thus would appear not to be economically feasible for purposes of the present invention.
In the Kerr-McGee patent, zinc granules gave a modest reaction in attempting selenium recovery. Copper, manganese, magnesium and aluminum powders gave very modest selenium reaction.
Mayenkar U.S. Pat. No. 4,565,633 discloses a process for removal of dissolved heavy metals from waste effluents. The process disclosed in the patent is somewhat similar to that of the Kerr-McGee patent. Mayenkar suggested the use of coarse iron filings (optimally 35 to 45 mesh, U.S. Standard Sieve), in a bed into which the aqueous solution was introduced. A long contact time with the iron filings was relied upon in the disclosed process. A pilot plant was actually built in accordance with the teachings of the patent, to treat Firebaugh irrigation effluent, but the project was unsuccessful.
In U.S. Pat. No. 4,026,797, nickel, cobalt and iron gave reasonable recoveries of selenium at 180 psi autoclaving, at pH less than 3. Sodium sulfide and 5 grams per liter iron as ferric sulfate, also with autoclaving, was reported as giving a reasonable selenium conversion.
In U.S. Pat. No. 4,497,654, chromous sulfate reduction was used in metal sulfate solutions to effectively reduce 10 milligrams per liter selenium at 60.degree. C. This would be unworkable, as the resultant chromium input would be almost as deleterious as the original selenium.
In U.S. Pat. No. 4,544,541, sodium borohydride at 5 grams per liter was effective in reducing 18 milligrams per liter of selenium down to about 1 milligram per liter. This could be optimized to be effective, but is much too expensive to be practical.
In refinery effluents, reverse osmosis is not seen as a workable process for separating selenium, because of oil and grease in the effluent water.
It is known that an iron hydroxide (ferrous iron) process has been tried with refinery water and has been found to work well for removing the selenite in the water column but not for removing selenate, elemental selenium or selenide.
An important object of the present invention is to efficiently and economically remove selenium (including selenate, selenite, elemental selenium and selenide) and other toxic substances from refinery waste water effluents and similar industrial effluents.